1 Field of the Invention
This invention relates to a front surface grinding machine and method of operating same, and more particularly to a machine for and method of grinding down the sides of very small pieces and in particular two pieces to be placed end to end, for instance the end surfaces of optical fibers mounted on connection holders for two multifiber cables.
2 Description of the Prior Art
To butt optical fibers end to end, several methods are known, among which note is made of the following:
(1) bonding fibers end to end by partial fusion of the ends, after the fibers have been brought together; PA0 (2) cutting fibers by a very slight preliminary break on each fiber, followed by traction; this method affords sufficient surface quality for a satisfactory connection quality when the fibers to be connected are laid end to end and an index adaptor agent is applied; PA0 (3) cutting fibers with no particular precaution, fixing in a holder generally by bonding, and manual polishing of the end faces constituting the fiber connection plane.
The first two methods are usually employed when the fibers must be connected one by one.
The present invention is particularly concerned with the third method. When several fibers are to be connected together, such as in the case of numerous fibers in two bundles or cables, advantage can be gained by simultaneously dealing with all the fibers at the same time. This method enables a substantial reduction in the individual handling for each pair of fibers and thus the time taken to connect them. This simultaneity in dealing with the fibers is achieved both for the unsheathing, positioning or bonding operations and for simultaneously aligning all the fiber tip ends so that they lie in the same plane.
To achieve this alignment, it is necessary to resort to a grinding machine usable under good conditions, especially for connecting optical fiber cables in situ. Experience has shown it unnecessary to undertake polishing of the opposite optical fiber ends. One fine grinding run is sufficient whilst doing the utmost to obtain excellent geometric definition.
Front surface grinding machines are described in U.S. Pat. No. 3,299,579 and U.K. Pat. No. 441,022. Such a grinding machine comprises an abrasive wheel, a wheelhead spindle, means for rotating the spindle, a single workpiece holder, and means for imparting reciprocating rotational motion to the holder about an axis parallel to that of the spindle; the holder motion is in a circular sector having ends wherein the holder is respectively in front, i.e., at the center, and to the side of the front working surface of the abrasive wheel.
According to U.S. Pat. No. 3,299,579, the workpiece holder is a rotating arm including a workpiece guidance recess. The means for imparting reciprocating rotational motion to the workpiece holder is a crank arm. Two superposed and coaxial circular abrasive wheels simultaneously grind opposite faces of a workpiece. Each abrasive wheel includes three concentric abrasive portions having different grades or hardnesses such that the piece is initially rough ground and then ground ever more finely as it passes from the circumferences towards the centers of the two wheels. During each grinding cycle, the workpiece holder recess receives the lowest piece in a stack of such pieces in a chute when the holder is on the side of the abrasive wheels; then the holder pivots towards the center of the two wheels to progressively grind the two workpiece faces; finally, the piece thus ground falls towards an endless belt conveyor through a central circular column, coaxial with the hollow spindle carrying the lower abrasive wheel. During grinding, the piece is not actually carried by the workpiece arm but is driven thereby. While the piece is rotated a guide directs it to the opposite working surfaces of the two abrasive wheels. The piece may even rotate on its own axis when it is being "rough" and "intermediate" ground by the outer peripheral and intermediate abrasive portions. In the other rotational direction, the workpiece holder pivots away from the center of the abrasive wheels towards the workpiece input stack, without carrying a workpiece.
In U.K. Pat. No. 441,022, the workpiece holder is a swinging arm that is subject to reciprocating motion between the outer circumference and the center of two coaxial and superposed abrasive wheels by a connecting rod, pivoted arm and eccentric path cam arrangement. The workpiece holder is in the form of a vise capable of gripping one or more pieces, such as bricks. In this case, the pieces are ground during the two reciprocating motions of the workpiece holder. The return motion, from the center to the edges of the abrasive wheels, scratches the brick workpieces; scratching is of no importance to the present invention since the workpieces are not bricks. If however the workpieces are fragile elements, such as optical fibers to be joined together, scratches on the fiber end surfaces give rise to deleterious properties. When the fibers are joined end to end the scratches introduce considerable optical signal transmission insertion loss.
The grinding method as per U.S. Pat. No. 3,299,579 partly solves this drawback since the pieces are only ground during the first rotational motion, from the edge towards the center of the two abrasive wheels. However, such a method cannot be employed for grinding the ends of optical fibers fixed onto a holder. First of all, such multifiber holders are not stackable and cannot fall by gravity onto the center of the abrasive wheels, as the fibers come from a bulky, difficult to handle cable, unlike small individual pieces. Secondly, the multifiber holder must be firmly fixed to the workpiece holder so as to obtain optical fiber alignment faces that are strictly coplanar and ground to a very high degree of accuracy. If such were not the case, this would cause misalignment and lack of parallelism of the optical fiber ends to be butted end to end.